Insulation breakdown of dry-type valve-side bushing cores under electrothermal compound stress has become a key factor limiting the safe and stable operation of the converter transformer. Based on slices of the real bushing core, it is observed for the first time that fold, peeling off, and fracture defects exist near the aluminium (Al) foil edge, accompanied by many metal spikes with a curvature radius <4 μm. Coupled with a capacitor screen thickness of ∼2 mm, an electrical tree model is designed to simulate insulation defects near aluminium foil edges inside dry-type bushing cores, whose insulation degradation characteristics under electrothermal compound stress are investigated accordingly. The results show that high temperature significantly increases the dielectric loss heat energy density and Joule heat energy density, accelerating the insulation degradation near Al foil edges. The parallel crepe paper interface significantly accelerates the growth of electrical trees under AC-superimposed positive DC voltage, while this effect is not significant under AC voltage. As the crepe paper consists of wooden fibres of 10 μm diameter, the electrical trees tend to grow along interfaces between fibres and epoxy resin to form “feathery” electrical trees in epoxy resin-impregnated paper samples.

中文翻译：

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电热复合应力下干式套管铁芯铝箔边沿绝缘劣化特性

干式阀侧套管铁芯在电热复合应力作用下的绝缘击穿已成为限制换流变压器安全稳定运行的关键因素。基于真实套管芯的切片，首次观察到铝（Al）箔边缘附近存在折叠、剥落和断裂缺陷，并伴有许多曲率半径<4 μm的金属尖刺。结合~2 mm的电容器屏蔽层厚度，设计了电树模型来模拟干式套管芯内铝箔边缘附近的绝缘缺陷，并据此研究了其在电热复合应力下的绝缘退化特性。结果表明，高温显着增加了介电损耗热能密度和焦耳热能密度，加速铝箔边缘附近的绝缘退化。平行皱纹纸界面在交流叠加正直流电压下显着加速电树的生长，而在交流电压下这种效果不显着。由于皱纹纸由直径为 10 μm 的木纤维组成，电树倾向于沿着纤维和环氧树脂之间的界面生长，从而在环氧树脂浸渍纸样品中形成“羽毛状”电树。